So Ahmadinejad — in his Christmas Day address to the British public — thinks Jesus would have been on his side. I have news for him. Jesus would no more have been on his side than the Holocaust never happened. Jesus was a patriotic Jew — not quite the freedom fighter Jews living under Roman occupation were hoping for but a prophet of the Jewish people, a devout believer in the Jewish God and stringent preacher of His law, a fierce ethical polemicist, a lover of peace but in so angry and ironic a spirit that it sometimes feels the opposite of peace. Maybe too Jewish in that case — but that is to make another point.

I argue this in a Channel 4 film to be shown this Sunday, Jesus the Jew. Forty years ago, while teaching English at King David High School in Manchester, I got into a spot of bother with my pupils’ parents for saying something similar. It had disturbed me that when Jesus’s name came up in the course of a class on John Donne or DH Lawrence — it doesn’t matter which: study English literature and you’ll hit upon Jesus — the overwhelming response was angry embarrassment. Jesus was not someone my pupils felt they should be asked to talk or even know about.

I don’t say I understood nothing of their feelings. Jews have crowded memories enough; there are some things we would rather forget. But Jewish or not, high-school children of 15 and 16 were culturally obliged, it seemed to me — obliged to themselves no less — to be informed about Jesus and not to be thrown into confusion by his name.

So I got them to write an essay on Jesus. “Over our dead bodies,” a number of the parents responded. A few rang the headmaster. If this was what was meant by a Jewish education they would have to consider taking their children elsewhere. Let them, was my attitude. Religion is no excuse for ignorance.

But the school, of course, thought differently. They were not going to allow a mass exodus on account of Jesus Christ. Not again.

Of the pupils who did write the essay, many rehashed the scurrilous defamations of Jesus which have circulated within Judaism for centuries. To keep it brief — Jesus (real name Yoshkie) was the son of a whore, a bastard who abused the rabbis assigned to teach him, engaged in all manner of blasphemy and lewdness, and was finally kicked out of yeshivah.

For what he went on to do by way of revenge on the Jewish people — ie invent Christianity (so be careful who you throw out of yeshivah) — he now sits for all eternity in boiling excrement.

That this is no more than answering like with like, given the libels Christians have visited on Jews for two thousand years, I accept. Christianity has been our calamity; it would be no surprise, all things considered, if we were to tell viler stories still. But it is depressing, nonetheless, to see us fail, when challenged to be serious, to rise above the ugly superstitious libels of those who have superstitiously libelled us.

Intellectually, we might just as well be sitting in boiling excrement ourselves. Not only does our reluctance to engage with Jesus shut us out from knowledge of other religions — and that was what I hated about the shrinking and revulsion I encountered at King David: the gating of ourselves in — it denies us access to a crucial chapter in the history of our own.

For no matter what has been done subsequently in his name, the truth is that Jesus spoke as a Jew, for Jews and to Jews — exclusively to Jews, he often claimed (“I am not sent but unto the lost sheep of the house of Israel”) — and was not only innocent of Christianity but would have railed against it.

“I believe with Jesus,” Martin Buber wrote, “but I don’t believe in him.” Believing “in” him is of course impossible for us, because believing in him means believing he was the son of God. This would have been impossible for Jesus, too. He never blasphemed against the One God by claiming to be his Son. And never claimed to be setting up a rival religion. A renewed Judaism was his goal, a Judaism re-affirmed according to the word of God, in fulfilment of His laws and in Messianic expectation of His coming.

We forget that the Messiah was a Jewish concept, and baptism a Jewish ritual. The words have grown alien to us by association, or, if you like, by misappropriation. I am not suggesting we go knocking on the gates of the Vatican and asking for our Jesus back. But we should be historians, at least, of our own wisdom. We need that wisdom today, but then when do we not?